Electric discharge tube and cathode therefor



April 14, 1959 a E. s. DORGELO ELECTRIC DISCHARGE TUBE AND CATHODE THEREFOR Filed April 2, 1956 INVENTOR EDUARD GERARDUS DORGELO fl y AGENT Fi.3f Fi.4

United States Patent i ELECTRIC DISCHARGE TUBE AND 'CATHODE THEREFOR Application April 2, 1956, Serial No. 575,642

Claims priority, application Netherlands April 2, 1955 4 Claims. (Cl. 313-273) The invention relates to an electric discharge tube, more particularly a transmitter tube, having a directly heated cathode, and more particularly to such a tube for use at high frequencies having a cage-shaped cathode.

Cage-shaped cathodes are known, which consist of a plurality of wires, divided into two groups, of which one group is wound on a right-hand helical line and the other on a left-hand helical line or part thereof. The wires of each group lie in a surface of revolution, for example a cylindrical surface, the surface of revolution of one of the two wire groups being located coaxially inside that of the other group, while the wires of the two groups are welded to one another at a few crossings. The wires are secured at each end to a supporting member, which is fixed in the tube. Since the wires lie helically on a surface of revolution, the supporting members need not be arranged so as to be movable, since the expansion of the wires upon becoming hot produces a greater or smaller curvature of the wires in their surface, so that no difficulties arise. However, as an alternative, one of the supports may be supported from the wires themselves. The supporting members are preferably shaped in the form of a metal annular disc, to which the cathode wires are secured.

Since the cathode wires do not lie in one surface, there is the disadvantage that the distance from the next-following grid is not the same for all wires. The difference in distance results in that two groups of electrons traverse the cathode-grid space with ditferent transit times. These groups of electrons do not co-operate and reduce the efficiency of the tube at high frequencies. This disadvantage is particularly great, if the distance between the cathode and the first grid is small, as is the case with tubes for use at high and very high frequencies.

According to the invention this disadvantage is substantially completely obviated, if in such a discharge tube only the wires lying in the surface of revolution nearest the next-following electrode have a satisfactory thermal electron emission.

The group of wires in the interior surface are preferably made of tungsten, tantalum or molybdenum, whereas the group of wires lying in the external surface is preferably made of one of these metals to which thorium is added. These wires may, however, be rendered satisfactorily emissive in a different way, for example by coating them with barium oxide.

The invention will be described more fully reference to a drawing, in which Fig. 1 is a side view vention;

Fig. 2 is a cross section of a transmitter tube having such a cathode;

Figs. 3 and 4 are cross-sections of two modifications of a cathode according to the invention.

of a cathode according to the in- 2,882,436 Patented Apr. 14, 1959 ice Referring to Fig. 1, reference numeral 1 designates a central supporting rod, to the end of which is secured a metal support shaped in the form of a disc 2. T o the other end of the cathode is secured a supporting ring 3. Between the disc 2 and the ring 3 provision is made of two sets of wires 4 and 5. The set 4 is wound in this case on a right-hand helical line and arranged in a cylindrical surface, surrounding the cylindrical surface of the wires 5 (see Fig. 2). The wires 5 are wound on a left-hand helical line. The wires of the two groups are welded to one another at a few crossings, for example at 8. All wires are traversed in parallel by the heating current. Only the wires 4 have a high thermal electron emission, whereas the wires 5 have a low thermal emission. As is evident from Fig. 2, only the distance between the grid 6 and the wires 4 determines the properties of the tube. The wires 5 serve mainly as supporting wires, which prevent the wires 4 from sagging. It is therefore possible to choose the number of wires 5 to be smaller than that of the wires 4', which is illustrated in Fig. 3. It is in general desirable also for the diameter of the wires 5" to be smaller than that of the wires 4", which is shown in Fig. 4.

The grid 5 is surrounded by an anode 7, which may form at the same time part of the tube wall.

The use of such a cage-shaped cathode has the advantage that a large emissive surface, and hence a high transconductance can be obtained.

Although a particular embodiment is described, it will be obvious that also other structures are possible within the scope of the invention. The tube may, for example, contain a plurality of grids and the thickness of the cathode wires may be different, so that the current consumption of the non-emitting wires is reduced.

What is claimed is:

1. A cathode for an electric discharge tube comprising a substantially cylindrical member constituted of two sets of coaxially-aligned wires wound in opposite senses, one of said sets lying within the other of said sets, the outer set of wires possessing a higher thermal electron emissivity than the inner set of wires.

2. In an electric discharge tube of the high frequency, transmitting type, a cathode comprising a substantially cylindrical member constituted of two sets of coaxiallyaligned, helically-wound, intercrossing wires wound in opposite senses, said wires of said two sets being secured together at least at some of their crossings, means for supporting opposite ends of said wires, and an electrode adjacent said cathode and closer to one of said sets of wires, the wires in said closer set having a higher thermal electron emissivity than the wires in the other set.

3. An electric discharge tube as set forth in claim 2 wherein the closer set of wires has an oxide coating thereon, and the other set of wires is uncoated.

4. An electric discharge tube as set forth in claim 2 wherein the electrode adjacent said cathode is a control grid which closely surrounds said cathode, and the outer set of wires has the higher electron emissivity.

References Cited in the file of this patent UNITED STATES PATENTS 1,596,672 Luckash Aug. 17, 1926 1,874,355 Round Aug. 30, 1932 2,210,761 Hennelly Aug. 6, 1940 2,363,028 Warnke Nov. 21, 1944 2,724,069 Young et a1. Nov. 15, 1955 

